Process for producing dielectric sheets



Patented Jan. 30, 1951 William J. Lies, Williamstown, Mas... assignor a Sprague Electric Company, North Adams,

Mm, a corporation of Massachusetts No Drawing. Application gully 12, 1947,

Serial No. 760,8 5

6 Claims. (Cl. 25-158) I I This invention relates to dielectric materials and more specifically refers to processes for producingvery thin dielectric sheets which predominate in high dielectric constant ceramic materials;

It has been known for many years that titanium dioxide possesses a dielectric constant far exceeding most other common dielectric materials of both organic and inorganic nature. For this reason its use as a dielectric material in certain types of electrical condensers has been extensive. One type which has been produced for high voltage applications consists of a disc of titanium dioxide of thickness of about .010 to about .100 inch. The diameter generally is provided with a collar to give it rigidity and to increase the flash over voltage. Condensers of this type are generally of quite low capacity and are usually employed in series for high voltage applications. Another'type which has been produced in quantity is the cylindrical or tubular type condenser wherein a very thin walled tube of titanium dioxide is extruded and fired to produce a rigid cylindrical body which can be provided with electrodes on the inner and outer surface. Condensers of this type have been used to replace mica condensers for low voltages, low capacity applications. Up until this time no practical method has been suggested for producing dielectric sheets of thickness less than about .010 inch.

The thinner sheets arerequlred for high capacity, minimum volume and maximum efiiciency in high or low voltage condensers.

Another method suggested for making thin films is described in US. Patent No. 2,360,479. In this patent is disclosed a method of producing relatively thin dielectric sheets by firing a preliminary film which is obtained by casting a suspension of titanium dioxide in a water solution of a water soluble polyvinyl resin, as represented by polyvinyl alcohol. Upon drying, the preliminary film contains from about 5% to about 6% of the polyvinyl alcohol. In order to make possible the manufacture of fiat films, the casting is done upon a fiat base which must be of-plastic such as polystyrene. When the fihn is dry, the whole assemb y, that is, film and base, is placed in a furnace and fired at a temperature sufiicient to sinter the particles of titanium dioxide together. At the same time the polyvinyl alcohol binder and the base section are decomposed and removed from the titanium dioxide, leaving only inorganic materials. While thin dielectric sheets may be obtained by this procedure, it is awkward and expensive since the polystyrene base is completely 2 used up without recovery and the furnace volume required is increased. More serious is the (hillculty that the dielectric sheet resulting from the process is no longer fiat and uniform. Unfortunately, the titanium dioxide particles apparently move somewhat during the removal of the resin in the firing process and as a resultno longer have the smooth fiat surface dimensions obtained when the preliminary film is compacted in a press. If the polystyrene base is not employed, although this procedure is not suggested, it has been found that the strength of the preliminary film per se is not sufficient to allow it to'be fired in a furnace without special supplemental ele- 20 dielectric sheets by new and improved processes.

A still further object is to produce new and im-' proved electrical condensers. Additional objects will become apparent from a consideration of the following description and claims.

These objects are attained in accordance with the invention wherein I employ a method for producing dielectric sheets which comprises firing a film consisting of a major portion of a compound selected from the class containing titanium dioxide and meta-titanates and a minor portion of a hydrophilic condensation resin. In a still more restricted sense this invention is concerned with a method for producing dielectric sheets which comprises firing at a temperature in excess of 1000 C. a film consisting of a major [portion of titanium dioxide and a minor portion 1360 C. a cast film of thickness less than about .010 inch, said film consisting of a predominant portion of titanium dioxide and a minor portion of a hydrophilic melamine-formaldehyde condensation resin. The invention is also concerned with electrical condensers utilizing the dielectric sheets produced as indicated above.

According to my invention I have discovered 4 I. that hydrophilic condensation resins exhibit unusual properties binders for particles of inorganic dielectric materials, particularly in casting slips. In contrast to other binders for dielectric materials, resins of this type do not have to be used with sweliable gums, peptizing agents, electrolytes or other modifying agents. They impart to the preliminary filmextremely desirable physical properties and can be introduced in high concentrations. The preliminary dielectric aims of the invention, with a major portion of dielectric particles and a minor portion of theresinous binder, are relatively tough and rigid and may be fired, without special support, at elevated temperatures to remove the binder without deformation, warping etc. of the resultant dielectric sheet. This desirable characteristic is in direct contrast with natural gums and rosins and polyvinyl alcohol, since films employing these binders require modifying agents and then support during firing.

Among the condensation resins which are useful in accordance with my invention are hydrophilic types of melamine-formaldehyde resins,

resorcinol-formaldehyde resins, phenol-formaldehyde resins and the like. In accordance with my invention these resins should be partially polymerized before use and must be hydrophilic in nature. As the suspending medium for the dielectric particles and as the solvent for the resin may bamentioned water and low molecular weight alcohols and glycols as well as mixtures thereof.

likcellentfilmsofminimumthicknesscanbe cast from suspensions of slips of the dielectric particles in a solution of the resinous material heretofore mentioned. The solvent may be removed from the film after casting by drying at elevated temperatures or by other means. It is preferable also to cure the resinto its full extent at a temperature above 100' C. The resultantcastandcuredfilmmaythenbefiredat elevated temperature to remove the resinous binder and sinter the particles of the dielec material. 1

Among the dielectric materials particularly adapted to the process disclosed herein are titanium dioxide and the metatitanates. The firing temperature to which the cast film is subiected is dependent largely upon the nature of the inorganic dielectric used and should-be sumcient to sinter the particles together to give a dense, rigid sheet. The preparation of the casting slip consists generally of mixing the finely ground ceramic particles and the resinous binder in the sus-' pending medium, for example, water. The sequence of steps is not too important since the ceramic particles can be mixed and suspended in the water before the resin is added. However, it is generally desirable to dissolve the rain in the water solvent before adding the titanium dioxide, since the resin decreases the viscosity of the water suspension to a great extent and the successfuicastingofverythinfilmsisiosome degree dependent upon this viscosity decrease. According to my invention 1 have thus found that the hydrophilic condensation resinous binder acts not only in its binding capacity but alsoasapeptizingagent. WhileIamnotfully aware of the reason, however, it appears that thisactionisduetothefactthatthepartially polymerized molecules of the resin act somewhat as an electrolyte. For this reason, it is preferabletoemploylowmolecule weighthydrophilie condensation resins for the casting slip although the molecular weight is, of course, increased during the curing of the cast and dried film. As a general rule I prefer to keep within the ratio of 1 to 20 parts of inorganic dielectric material to 1 part of the resinous binder, based, of course. upon dry weight. For optimum results I have found that a concentration of about 7 parts of inorganic dielectric material to 1 part of resin is desirable. The amount of solvent and suspending material employed is generally between aboutltoaboutlopartstolpartofresin. In order to obtain casting slips with low viscosity and to obtain thin cast films, it is not necessary to use very high concentration of water in conjunction with the novel binding materials of the invention.

The slip produced in accordance with the invention may be cast in the usual manner and any type of form or mold may be employed in the manufacture of thin dielectric sheets. In accordance with one of the preferred embodiments of this invention a polished and smooth fiat base surface is desirable. For example, a stainless steel belt is useful. Other polmhed surfaces such asbrasaglass and rubber maybeused. Ifso desiredathinoilorgreasefilmmaybeplaced upon the casting surface before application of the slip thereto. However, this is not always necessary.

Thecastingmaybeaccomplishedsimplyby pouring the slip upon the smooth surface and allowing it to spread out by gravity. It is possible to accelerate the broadening by means of doctor blades and the like. Agitation of the base materialduring the castingprocess is also useful in obtaining films of minimum thickness.

I have found also that extremely thin films may be obtained by maintaining the cast slip under reduced pressure during the agitation. The resultant film is free of bubbles and will possess a very smooth surface.

Aftercastingoftheillmitisallowedtodry by convection of air or it may be subjected to elevawd temperatures. After the rying is complete the film is subjected to elevated temperatures sufficiently high to complete the P01!- merisation of the hydrophilie condensation resin binder. This temperature is generally above C. and in the neighborhood of C. for the preferred resins of the invention. If so desired a polymerimtion accelerator may be added to the suspension in order to speed the curing of the resin.

In accordance with one of the limited embodiments of the invention, it is possible to compact the film between heated platens in a press. in order to obtain maximum density of the preliminary film. This is generally unn since the film is inherently smooth but may be incorporatedintheprocesstoreplacetheresin curing since, under pressure, the resin curing cycle is decreased. Likewise. polymerization catalysts may be added to the The preliminary film thus obtained is tough. fiat, self-supporting and adapted to the firing operation. The thickness of the film is generally less than .0l0'and may be lower than .005 inch. The following example is representative of the process described herein. 350 grams of titanium dioxide powder of ilnenus greater than 325 mah wasmixedwithlOOcaofwatertogiveavery thick paste. 50 grams of a water-soluble. partialiy polymeriseo melamine-formaldehyde resin (sold commercially under the trade name Melmac M405) was added to the paste, upon further agitation went into solution and the viscosity of the paste decreased markedly to give a fiuid slip. grams of glycerin was added, to lower the drying rate. The free-flowing castin slip thus prepared was poured upon a stainless steel plate which had been coated with a thin layer of beeswax. The cast film was carefully dried. After most of the solvent had been removed by air drying, the film was cured for 30 minutes at 150 C. The film which was then removed from the stainless steel base, was tough and retained the fiat, smooth surface of the base material. It was placed in a furnace which was heated to a temperature of 1360 C. where it was held for 30 minutes. The film was removed from the furnace after it had cooled to 1200 C. Upon cooling to room temperature a rigid, flat thin sheet of titanium dioxide was obtained. The thickness of this sheet was .007 inch. A portion of each side of the sheet was provided with a fired silver electrode surface, producing a simple electrical condenser. This condenser was tested at 1 megacycle and gave a Q value of 820. From the known area of the silver electrode and thickness of the dielectric sheet, it was possible to calculate the dielectric constant of the material. This was found to be 108.

While my invention has been directed particularly to the preparation of dielectric sheets utilizing hydrophilic condensation resin binders, I also contemplate that other binding materials .can be used, if they are available with such molecular weights and such chemical structure that they will exhibit the peptizing properties described herein. Polyvinyl alcohol, for example.

does not fall within this category and films employing it as a binder are not satisfactory in many respects.

To obtain films of minimum thickness, it is advisable to treat the cast film before it is dried by a special process which combines agitation with reduced pressure. Thereby, it is possible to remove all traces of bubbles and to obtain a uniform plane surface on the upper side of the While my invention has been particularly directed to the production of thin dielectric sheets, the casting slips which employ the resins described herein are useful in many other fields in which slip casting is generally employed.

For example, it is possible to slip cast intricate shapes in metal, plastic, or rubber molds. Heretofore, plaster of Paris molds were used to absorb the water from the slip. As a result numerous molds were required and the surface of the cast object reflected the many imperfections in the relatively delicate surface of the plaster of Paris. After casting, the mold can be heated carefully to remove the moisture from the slip, after which time the dried object may be removed from the mold and fired in the usual manner.

As many apparently widely different embodiments 01 this invention may be made without departing from the spirit and scope hereof, it is to be understood that the invention is not limited to the specific embodiments hereof except as defined in the appended claims.

What I claim is:

1. In a method of producing dielectric sheets having thicknesses of the order of 10 mils, the steps of casting a thin film from a slip of finely divided dielectric particles of a compound selected from the class consisting of titanium dioxide and titanates, which slip includes a partially polymerized hydrophilic condensation resin in an amount of from 5 to 50 per cent by weight of the dielectric particles, drying and curing the cast film to substantially completely polymerize the resin, and then firing the cured film to burn off the resin and sinter the dielectric particles together.

2. The method as defined by claim 1 in which the cast film, before curing, is agitated and subjected to reduced pressure to smoothen its upper surface.

3. A method for producing thin dielectric sheets which comprises firing a thin film of about 20 parts to about 1 part by weight of finely divided particles of the class consisting of titanium dioxide and titanates, said particles being bonded together by about one part by weight of a cured hydrophilic condensation resin, and continuing the firing till said particles are sintered together and the resin is burnt away.

4. A method for producing thin dielectric sheets which comprises firing a thin film of about 1 to 20 parts by weight of titanium dioxide particles bonded together by about 1 part by weight of a cured hydrophilic melamine-formaldehyde resin to burn away the resin and sinter said particles together.

5. A method for producing thin dielectric sheets which comprises casting a thin film from a slip containing a predominant portion of finely divided titanium dioxide particles, a minor portion of a partially polymerized hydrophilic condensation resin and water, drying ,the film, curing the resin in the dried film substantially completely and thenfiring the film to burn oil the resin and sinter the titanium dioxide particles together.

6. A method for producing thin dielectric sheets which comprises casting a thin film from a slip containing a predominant portion of finely divided titanium dioxide, a minor portion of a partially polymerized hydrophilic melamine-formaldehyde resin and water, drying the film, curing the dried film to substantially completely polymerize the resin and form a self-sustaining plate and firing the plate at a temperature of about 1250 C. to about 1360 C. for about 30 minutes to remove the resin therefrom and sinter the titanium dioxide particles together.

WILLIAMJ.LIES.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 2,434,236 Verwey et a1. Jan. 6, 1948 

1. IN A METHOD OF PRODUCING DIELECTRIC SHEETS HAVING THICKNESSES OF THE ORDER OF 10 MILS, THE STEPS OF CASTING A THIN FILM FROM A SLIP OF FINELY DIVIDED DIELECTRIC PARTICLES OF A COMPOUND SELECTED FROM THE CLASS CONSISTING OF TITANIUM DIOXIDE AND TITANATES, WHICH SLIP INCLUDES A PARTIALLY POLYMERIZED HYDROPHILLIC CONDENSATION RESIN IN AN AMOUNT OF FROM 5 TO 50 PER CENT BY WEIGHT OF THE DIELECTRIC PARTICLES, DRYING AND CURING THE CAST FILM TO SUBSTANTIALLY COMPLETELY POLYMERIZE THE RESIN, AND THEN FIRING THE CURED FILM TO BURN OFF THE RESIN AND SINTER THE DIELECTRIC PARTICLES TOGETHER. 